During previous experimentation, one of the present inventors successfully designed and operated a passive magnetic bearing in which loops on a rotor travelled in a circular path through magnetic fields created by a circular array of stationary permanent magnets. Circumferentially adjacent poles of the magnets had opposite polarities. Each loop had outboard and inboard portions which each travelled successively through a respective set of oppositely directed magnetic fields. For optimum performance, the lengths and electrical characteristics of the loops were custom designed for different installations. The optimum loop lengths depended on the corresponding lengths of the magnets, and the loop inductances and resistances were selected to provide a desired degree of stiffness at the design angular velocity of the bearing. When a bearing of this type used permanent magnets exclusively, its stiffness was dependent on the angular velocity of the bearing, so its stiffness was quite low at low velocities. Energy losses were experienced due to the inhomogeneous character of the magnetic fields.
A bearing according to the present invention has certain advantages over the bearings described in the preceding paragraph. According to one feature of the invention, the fields are created by alternating current so the satisfactory operation and stiffness of the bearing is not exclusively dependent on the inductance or resistance of the loops. The frequency and current supplied to the windings of the electromagnet are variable so that adjustments can be made to optimize the bearing's operation without reconfiguring the hardware. The stiffness of the bearing is not substantially dependent on or affected by the angular velocity of the bearing.
Unlike some previously proposed magnetic bearing systems, the bearings of this invention are not dependent on rotational movement. This bearing will even maintain the rotor at a desired position when all components are stationary.
Fine tuning of a magnetic bearing can be difficult, and it sometimes has required the rewinding of the rotor loops to change their inductive reactances and other electrical characteristics. To solve this aspect of fine tuning, capacitors are wired in series with the loop windings. By changing the capacitors or their capacitances, the loops may be given optimum electrical characteristics for a particular bearing installation.
In another respect, in bearings according to the invention, the magnetic fields are homogeneous to minimize energy losses. The fields have constant polarities along the loop path, so they operate effectively with loops of differing lengths.
Bearings constructed according to this invention can take many forms, some of which are shown and described below.